How to Impound Cars in "Crackdown"

1). Visit the Agency Garage to view a list of the cars that you need to collect.
2). Locate a car on the list. If the car is unoccupied, break in and start it up. If the car is occupied, hijack the car using any method you desire.
3). Wait for the on-screen notification to appear, which will notify you that the car is, indeed, on the impound list. If the notification does not appear, step out of the car and try again.
4). Drive the car back to the Agency Garage.
5). Park the car in the center of your garage and exit the vehicle. The car will automatically be impounded, as evidenced by an on-screen notification.